Design of infrared camouflage cloak for underground silos

Luo Zhang,Qiang Zhang,Hong Ye

Department of Thermal Science and Energy Engineering,University of Science and Technology of China,Hefei 230027,PR China

Keywords:Underground silo Infrared camouflage cloak Imitative layer Insulation layer

ABSTRACT The temperature difference between the exposed surface of an underground silo and the surrounding soil surface is significant,which means a silo can be easily found by infrared detection.We designed an infrared camouflage cloak consisting of an imitative layer and an insulation layer for the silos.The imitative layer is used to imitate the thermal response of the soil to the surrounding environment.The insulation layer is used to weaken the impact of the internal temperature field of the silo on the lower boundary of the imitative layer.A silo model including surrounding soil and a soil model without silo were established,and the influences of the material and thickness of each layer on the infrared camouflage effect were analyzed.The results show that when using a silicone rubber containing alumina powder with a volume fraction of 3.18%as the imitative material,its thermal inertia is in consistent with that of the soil.Meanwhile,it was found that the thickness of the imitative layer doesn’t need to be greater than its thermal penetration depth to achieve the infrared camouflage,and the absence of the insulation layer will cause hot spots on the silo surface in winter to weaken the camouflage effect.The optimized thicknesses of the imitative layer and the insulation layer are 22 cm and 4 cm respectively.The simulations indicate that with the application of the cloak,the maximum value of the absolute values of the temperature differences between the average temperatures of the silo surface and the surrounding soil surface temperatures drops from 1.59°C to 0.31°C in summer and from 1.92°C to 0.21°C in winter.This designed cloak can achieve an all-weather and full-time passive infrared camouflage.?2020 China Ordnance Society.Production and hosting by Elsevier B.V.on behalf of KeAi Communications Co.This is an open access article under the CCBY-NC-ND license(http://creativecommons.org/licenses/by-nc-nd/4.0/).

1.Introduction

For underground silos,infrared reconnaissance and guidance technology poses a serious threat to their survival.Infrared camouflage technology can reduce infrared radiation contrast(typically in atmospheric windows:3-5μm and 8-14μm)between the target and the background,thereby reducing the probability of being recognized by the infrared detection.The infrared radiation of an object is mainly determined by its surface temperature and infrared emissivity.Because the exposed surface of a silo and its surrounding soil surface are at the same level,covering a thin layer of soil can make its infrared emissivity consistent with that of the surrounding soil surface.How ever,it is quite difficult to maintain the silo surface and the soil surface at the same temperature.The first reason is that the cover of the silo and the soil are different in thermophysical properties,thus the silo surface and the soil surface have different thermal response to the same environmental conditions.The second reason is that the temperature inside the silo needs to be maintained at approximately 20°C to ensure a normal operation of the equipment inside,while the temperature of the surrounding soil changes periodically.Therefore,infrared camouflage technology is needed to reduce the difference in infrared radiation between the silo surface and the soil surface.Currently,most researches focus on infrared camouflage for maneuvering targets.Due to the high temperature of internal power equipment,their surface temperatures are usually higher than those of the backgrounds.The corresponding methods are reducing surface temperature[1,2]and infrared emissivity[3-6].However,the surface temperature of the underground silos usually fluctuates around the background temperature.Thus,simply lowering the surface temperature and infrared emissivity cannot achieve infrared camouflage.It is necessary to control the surface temperature and infrared emissivity to merge their infrared radiation characteristics with that of the background.

In the control of infrared emissivity,VO2with a thermochromic effect has been extensively studied[7-9],other methods include:using the electrochromic effect of polyaniline[10,11],changing the bias voltage applied on graphene[12],utilizing the phase transition of Ge2Sb2Te5between crystal and polycrystal[13]and the UV sensitive property of Zn O[14].How ever,the infrared emissivity of a soil is usually above 0.9.In the above methods,the upper limits of the emissivity variation ranges are all below 0.9,which means they can only make the infrared emissivity of the target lower than that of the background.For situations where the target temperature is lower than the background temperature and the infrared emissivity of the target needs to be higher than that of the background,these methods are powerless.Therefore,infrared camouflage of underground silos cannot be realized by controlling the infrared emissivity.

Covering a thin layer of soil on the silo surface can make its emissivity the same as that of the soil surface,hence infrared camouflage can be realized by controlling the silo surface temperature to match the soil surface temperature.Kim et al.[15]used thermoelectric units to control the target surface temperature actively.They optimized the temperature by minimizing the infrared radiation contrast between the target and the surrounding environment in the thermal image.The experimental results show ed that this method can reduce 95%and 99%infrared radiation contrast in daytime and nighttime,respectively.How ever,this method requires the use of a thermal imager to observe the desired camouflage target in real time,and it is difficult to find a suitable location for the thermal imager for an underground silo.In addition,due to the large area of the silo’s exposed surface,the power consumption required to control its temperature can be very high.In terms of passively controlling temperature to achieve infrared camouflage,a large number of studies have utilized transformation thermotics[16-18]and scattering cancellation[19,20].These methods can thermally camouflage an object by manipulating the temperature profile around it.How ever,the temperature field of the target itself is still clearly identifiable,and the infrared camouflage cannot be achieved.Li et al.[21]designed a structured thermal surface that encloses the target placed on the background;Wang et al.[22]proposed an effective medium theory in thermotics by considering anisotropic layered/graded structures,and designed a two-dimensional structure that can hide the target’s temperature field.Both of the structures can hide the temperature field of the target.How ever,the background thermal field in their design is a unidirectional thermal field perpendicular to the view direction.They are unsuitable for underground silos for which the thermal environments are more complicated and the heat conduction heat flow directions are roughly parallel to the view directions.

The researches by Ye et al.[23,24]provide a feasible idea for infrared camouflage of underground silos.They numerically studied the relationship of the temperature difference between a target surface composed of an imitative material and the concrete road surface and their physical properties.The results show ed that under periodic environmental conditions,when the thickness of the imitative material is greater than its thermal penetration depth(δp=in which k,ρ and cpare thermal conductivity,density and specific heat capacity respectively,and ω=2π·（1/T）stands for the variation frequency of the ambient condition)and its thermal inertiais in consistent with that of the road,the target and road surface temperatures can be identical all the time.According to their studies and the reasons for the existence of the temperature difference between the silo and the soil surface analyzed above,we designed an infrared camouflage cloak composed of an imitative layer and an insulation layer from top to bottom.The imitative layer is used to imitate the thermal response characteristics of the soil to the surrounding environment,and its thermal properties are determined by the studies of Ye et al.[23,24].The insulation layer is used to weaken the influence of the internal temperature field of the silo on the lower boundary of the imitative layer.The infrared camouflage cloak is expected to make the silo surface temperature and the soil surface temperature consistent all the time,thereby achieving an all-weather and fulltime passive infrared camouflage.

In this work,an infrared camouflage cloak consisting of an imitative layer and an insulation layer for underground silos was designed.A silo model including the surrounding soil and a soil model without the silo were established.The composite material of alumina powder and silicone rubber was selected as the imitative layer material,and the volume fraction of the alumina powder was adjusted to make its thermal inertia consistent with that of soil.The thickness of each layer of the cloak was optimized according to the maximum value of the absolute values of the differences between the average temperatures of the silo surface and the soil surface temperatures and the maximum value of the standard deviations of the silo surface temperatures.Finally,the changes over time of the average temperatures of the silo surface and the soil temperatures and the temperature differences between them before and after the application of the optimal cloak were analyzed in different seasons.

2.Model

The structures of the underground silo and the infrared camouflage cloak are shown in Fig.1.The underground silo is simplified into a cylinder with a length of 30 m and an inner diameter of 5 m.The silo wall is made of 2 m thick concrete and the cover is made of 1.5 m thick concrete.The target in the silo is simplified into a cylinder of 24 m in length and 4 m in diameter.The target is concentric with the silo and its top surface is 3 m below the silo’s exposed surface.The internal material of the target is a kind of fuel and its surface is made of 1 mm thick Al alloy.The soil around the silo is 40 m deep and the outer radius is 20 m.The infrared camouflage cloak is mounted above the silo cover.In order to reduce the impact of the silo on the surrounding soil surface temperature,the diameter of the cloak is expanded into 15 m.To ensure that the silo surface and the surrounding soil surface are still at the same level,the thickness of the cover is reduced accordingly to make that the total thickness of the cover and the cloak is 1.5 m.In addition,a liquid thermal control system[25]is used to control the temperature inside the silo.To ensure a good contact with the temperature control coil,the inner wall of the silo is covered with 1 mm thick Al alloy,and the coil is welded around it.The total length of the temperature control coil is 43 m.The pipe diameter of the coil is 60 mm,and it’s made of 1 mm thick Al alloy.The portion welded on the peripheral surface is spring-shaped with a pitch of 1.42 m,and those welded on the end faces are helical with a pitch of 0.46 m.

According to the investigations of Ye et al.[23,24],the thermal inertia of the imitative material needs to be consistent with that of the soil,and its thickness needs to be greater than its thermal penetration depth to make its surface temperature consistent with the soil surface temperature.How ever,it is difficult to find a material with the same thermal inertia as that of the soil directly.Therefore,we chose a composite of alumina powder and silicone rubber[26]as the imitative material,and the same thermal inertia as that of the soil can be achieved by adjusting the volume fraction of the alumina powder.According to the Maxwell-Eucken 2 model[27],the thermal conductivity of the imitative material can be calculated as

where φ is the volume fraction of the alumina powder,and subscripts e,1 and 2 represent the imitative material,silicone rubber and alumina pow der,respectively.The density and specific heat capacity of the imitative material are

Fig.1.Schematics of the underground silo and the surrounding soil and the infrared camouflage cloak.

and

Therefore,the thermal inertia of the imitative material Peis

Through calculation,when the volume fraction of the alumina powder is 3.18%,the thermal inertia of the imitative material is approximately equal to that of the soil.The physical properties of the materials used in the simulations are given in Table 1,which includes the calculated physical properties of the imitative material with a thermal penetration depth of 35 cm.The insulation material is polyurethane.In addition,in the simulations,we assumed that the radiation properties of the concrete and the imitative material are in consistent with that of the soil,which can be achieved by covering the exposed surface of the silo with a thin layer of soil.

The finite element thermal analysis software I-DEAS[28]was used in the simulations.In the silo model,the following assumptions were made:the material is isotropic and the thermal properties are constant,the contact thermal resistances can be neglected,the thermal resistance of the coil wall can be ignored.The exposed surfaces of the silo and the soil convect with the ambient air,exchange heat by radiation with the sky,and receive solar irradiation.The convective heat transfer coefficient h can be given as[29]:

where V is the wind speed,m/s.The circumferential face of the soil is adiabatic.The water exchanges heat with the inner wall of the silo by convection.The inner wall of the silo and the target surface exchange heat via radiation,and both convect with the air inside the silo.The water in the coil flow s in from the top of the silo and flows out from the bottom.The inlet temperature is 20°C,the pressure difference between the inlet and the outlet is 20 k Pa,and the flow rate is 4.78 m3/h.Xining City in China was selected as a typical area,where the ground temperature at 40 m deep does not change with time,so the bottom surface temperature of the soil was set as 10.6°C[30].July 20 which has the highest daily average temperature was selected as a typical summer day,and January 9 which has the low est daily average temperature was selected as a typical winter day.The variations of the environmental parameters with time in each season are shown in Fig.2,including ambient airtemperature,sky temperature,w ind speed and total horizontal irradiation.All parameters were taken from the data of typical meteorological years[31].The initial temperature was set as 10.6°C.Moreover,to ensure that the soil temperature used for evaluation of the infrared camouflage effect is not affected by the silo,we modeled the exposed surface temperature of the soil additionally.The model size is 1 m(length)×1 m(w idth)×40 m(depth),the material is soil,and the boundary conditions and calculation conditions are exactly the same as that of the silo model.Because the soil model is adiabatic at the circumferential face,it is equivalent to a one-dimensional model along the depth direction.

Table 1 Physical properties[32].

Fig.2.Environmental parameters versus time in each season.

3.Results and discussion

3.1.Optimization of the thickness of each layer

The effect indicators of the infrared camouflage are the maximum value of the absolute values of the temperature differences between the average temperatures of the silo and soil surface temperatures(|ΔTsilo-soil|max)and the maximum value of the standard deviation of the silo surface temperatures(σT-silo，max).The former characterizes the closeness of the temperatures between the silo and soil surfaces,while the latter characterizes the uniformity of the silo surface temperatures.To find the optimized thickness of each layer,we fix the insulation layer thickness andoptimize the imitative layer thickness first.Then,we optimize the insulation layer thickness using the optimized imitative layer thickness.Finally,we change the imitative layer thickness again based on the optimized thickness combination.In order to reduce the amount of simulations,we first carried out a preliminary optimization at intervals of 10 cm,and then completed the final optimization at intervals of 1 cm.

Table 2 Infrared camouflage effects of the infrared camouflage cloaks in the preliminary optimization.

The results of the preliminary optimization are shown in Table 2.It can be seen that when the insulation layer thickness is fixed as 40 cm and the imitative layer thickness is reduced from 40 cm to 20 cm,both indicators decrease in the two seasons.When the imitative layer thickness is further reduced to 10 cm,both indicators become larger.Therefore,the preliminarily optimized imitative layer thickness is 20 cm.How ever,according to the studies of Ye et al.[23,24],the thickness of the imitative layer needs to be greater than its thermal penetration depth,i.e.,35 cm,which is inconsistent with the preliminary optimization.This is because in their researches,the bottom surfaces of the models are adiabatic.When the thicknesses of the soil and the imitative material are greater than their respective thermal penetration depths,the thickness of the material in both models are thermally in finite,i.e.,temperatures are constant at a certain depth.The simulation results of our soil model show that the soil temperature is constant(10.6°C)below 10 m depth,i.e.,its thickness is thermally in finite.

Fig.3.Temperature fields of the silo surface and surrounding soil surface at 14:00 in different seasons.

When the thickness of the imitative layer is greater than its heat penetration depth and the lower surface of the cloak is adiabatic,the upper surface temperature of the insulation layer would be maintained at a fixed value.However,the lower surface temperature of the insulation layer would reach approximately 20°Cdue to the temperature inside the silo.Thus,there would be a temperature difference between the upper and lower surfaces of the insulation layer.Through heat conduction,the upper surface temperature of the insulation layer would deviate from that fixed value,thereby affecting the silo exposed surface temperature.Therefore,making the imitative layer thickness greater than its thermal penetration depth cannot reach the thermally in finite effect,and the surface temperatures of the silo and the soil cannot be completely consistent.

It can be seen from Table 2 that when the imitative layer thickness is fixed as 20 cm and the insulation layer thickness is reduced from 40 cm to 0 cm,|ΔTsilo-soil|maxdecrease in both seasons,while σT-silo，maxare small and remains nearly unchanged,except when there is no insulation layer in winter.The uniformities of the silo surface temperatures are different in summer and winter.This is because that solar radiation in winter is weaker,and the temperature inside the silo has a relatively greater influence on its exposed surface temperature.When there is no insulation layer in winter,this influence is even stronger.Since the temperature inside the silo is higher than the surrounding soil,the center temperature of the silo surface will be significantly higher than the edge temperature.Therefore,σT-silo，maxwill be significantly greater in w inter than in summer.The large σT-silo，maxin winter means that the temperature uniformity of the silo surface is unsatisfactory,so the insulation layer needs a thickness of 10 cm to maintain good temperature uniformity of the silo surface in different seasons.Considering that approximately 0.01°C change of σT-silo，maxhas little effect on the uniformity of the silo surface temperature,here we take|ΔTsilo-soil|maxas the main indicator.Thus,the optimized thickness of the insulation layer is 10 cm.When the insulation layer thickness is fixed as 10 cm and the imitative layer thickness is changed from 20 cm to 30 cm and 10 cm,the two indicators in both seasons are larger than those of the cloak“Im20&In10”.Therefore,the cloak“Im20&In10”is the preliminarily optimized one.

The results of the fine optimization are presented in Table 3.As can be seen,when the insulation layer thickness is fixed as 10 cm,|ΔTsilo-soil|maxreaches a minimum when the imitative layer thickness is22 cm,and σT-silo，maxis approximately 0.2°C.Therefore,the optimized imitative layer thickness is 22 cm.When the imitative layer thickness is fixed as 22 cm,|ΔTsilo-soil|maxreaches a minimum when the insulation thickness is 4 cm considering both seasons,and σT-silo，maxis also approximately 0.2°C.It is worth noting that in summer when the insulation layer thickness is 0 cm,both indicators reach the minimum values.Thus,if only the summer condition is to be concerned,the insulation layer is not needed.When the insulation layer thickness is fixed as 4 cm and the imitative layer thickness is changed to 23 cm and 21 cm,|ΔTsilo-soil|maxare larger than that of the cloak “Im22&In4”.Therefore,the cloak“Im22&In4”is the finely optimized one.

The temperature fields of the silo surface and surrounding soil surface at 14:00 are shown in Fig.3,including the results of no cloak,cloak“Im22&In0”and“Im22&In4”.The time 14:00 is chosen because|ΔTsilo-soil|maxoccurs at this time both in summer and winter when using the cloak“Im22&In4”.As can be seen from Fig.3,before the application of the cloak,the silo surface temperatures in both seasons are significantly higher than that of the surrounding soil,and there are obvious hot spots in the center of the silo surface.After application of the cloak“Im22&In0”,in summer,the temperature difference between the silo surface and the surrounding soil surface is small,and there is no obvious hot spot on the silo surface.In w inter,the temperature difference between the edge of the silo surface and the surrounding soil surface is also small,but there is an obvious hot spot in the center of the silo surface.After application of the cloak“Im22&In4”,in both seasons,the temperature differences between the silo surface and the surrounding soil surface are small and there is no obvious hot spot.These results are in consistent with the previous analysis,and show that there is an obvious hot spot in w inter without the insulation layer,which further confirms that the cloak“Im22&In4”is optimal.

3.2.Infrared camouflage effect throughout a day

The average temperature of the silo surface,the soil surface temperature and the temperature difference between them with or without the optimal infrared camouflage cloak in different seasons are shown in Fig.4.As can be seen,without the optimal cloak,there is a significant deviation between the average temperature of the silo surface and the temperature of the soil surface.This is because the thermal inertia of the concrete is greater than that of the soil,and the response of its surface temperature to the changes of the thermal environment is more slowly.How ever,after application of the optimal cloak,the average temperature of the silo surface becomes almost consistent with the temperature of the soil surface.

Moreover,as can be seen from Fig.4,without the optimal cloak,the temperature difference fluctuates greatly and is approximately sinusoidal in summer.Before 14:00,the silo surface temperature is lower than that of the soil.After 14:00,the reverse is observed and the maximum temperature difference is 1.59°C,which occurs at 17:00.In winter,because the temperature inside the silo is higher than that of the surrounding soil,the silo surface temperature is higher than that of the soil surface for most time,and the difference is maintained at approximately 0.7°Cat night.After sunrise at 8:00,the surface temperatures begin to rise.Because the thermal inertia of the concrete is greater than that of the soil,the silo surface temperature increases at a slower rate.As a result,the temperature of the soil surface begins to approach the silo surface temperature,the temperature difference decreases gradually.Then,the soil surface temperature exceeds the silo surface temperature quickly,and the temperature difference becomes negative.At 11:00,the temperature difference decreases temporarily due to the sudden weakening of the solar radiation.During 11:00-12:00,the temperature difference continues to decrease.After 12:00,due to the superposition of the heating delay effect and the internal high temperature,the increase rate of the silo surface temperature begins to be higher than that of the soil surface temperature.Thus,the temperature difference increases from-0.82°C at 12:00 to a maximum temperature difference of 1.92°Cat 17:00.After sunset at approximately 17:00,the temperature difference decreases gradually to approximately 0.7°C.After application of the optimal cloak,the temperature difference is basically maintained around 0°C and the fluctuation range is small.The maximum temperature difference occurs at 14:00 in both seasons,and is 0.31°C and 0.21°C in summer and winter,respectively.

Fig.4.Average temperature of the silo surface,the soil surface temperature and the temperature difference between them with or without the optimal infrared camouflage cloak in different seasons.

4.Conclusions

We analyzed the reasons why the exposed surface temperature of the underground silo is different from the surface temperature of its surrounding soil,and designed an infrared camouflage cloak with an imitative layer and an insulation layer from top to bottom.The composite material composed of alumina powder and silicone rubber was selected as the imitative layer material.The volume fraction of alumina powder was determined to be 3.18%to make its thermal inertia consistent with that of the soil.It was found that the thickness of the imitative layer does not need to be greater than its thermal penetration depth to achieve a good infrared camouflage effect,and the absence of the insulation layer may result in hot spots on the silo surface in w inter and affect the infrared camouflage effect.The cloak“Im22&In4”is optimal.Before application of the optimal cloak,|ΔTsilo-soil|maxboth occur at 17:00,and are 1.59°C in summer and 1.92°C in w inter.After application of the optimal cloak,|ΔTsilo-soil|maxboth occur at 14:00,and are 0.31°C in summer and 0.21°C in winter.

Declaration of competing interest

We here declare that there is no conflict of interest.

Acknowledgement

This work was funded by the National Natural Science Foundation of China(contract grant number 51576188).